1. Field of the Invention
The invention relates to a shaft furnace e.g. a blast furnace, having a metal shell, a refractory furnace lining inside the shell and a taphole construction comprising an aperture in the furnace shell and a refractory plug located in the aperture, a drilled taphole extending through the plug and the lining.
2. Description of the Prior Art
A taphole construction as described above is conventional in the blast furnaces for the production of molten iron, although, in principle, it can also be used for other types of metallurgical furnaces. The shape of the aperture in the furnace shell (also called armour) may vary, depending on the structure of the furnace itself. As a rule, the entire taphole construction is of larger design for larger blast furnaces with a larger tapping capacity. Often, the shape of the aperture in the furnace shell is generally rectangular or is rectangular with an arch-shaped curve at the top. In the practice of the present invention, no specific shape is, however, essential. Usual dimensions of the width and the height of the aperture respectively vary between 40 and 60 cm for the width and 80 and 120 cm for the height. As a rule, the armour plate of the metal shell at the location of the aperture is provided with a collar, projecting to the outside, in which a plug is installed. In many constructions, the taphole is drilled so as to slope downwards from the outside inwardly for practical reasons.
Blast furnaces are operated under overpressure, which means that they can be tapped only periodically. To effect a tapping, the taphole is opened. Subsequently the taphole is closed again after tapping has been completed by inserting a hardened taphole mass e.g. clay into the hole. This mass has to be drilled, burned or pushed out for a subsequent tapping. Such taphole masses, therefore usually consist of a refractory component and a hardening binder component. During filling of the taphole opening, the temperature of the taphole wall is of importance. For, if this temperature is too high, the danger exists that volatile components in the binder evaporate almost explosively, and hamper closing of the hole. If, on the other hand, the temperature of the taphole is too low, the binder does not harden fast enough resulting in loss of operating time of the furnace or danger of opening of the taphole prematurely. As a rule, therefore, it is sought that the temperature of the taphole wall during operation, i.e. after several tappings, adopts a value within a temperature range of 500.degree. C.-800.degree. C.
With an embodiment of the taphole construction in which the refractory bricks, as is usual, consist of chamotte or equivalent material, after some time the problem of cracking in the taphole is experienced, or of crumbling of the taphole brick. This is caused by the strong fluctuations in temperature during and after tapping and the only limited resistance of the material used to temperature fluctuations. As the taphole brick in most constructions is situated partly behind the furnace shell, replacement of a defective taphole brick is not possible other than during a major repair of the furnace. Therefore, efforts have been made to achieve interim repairs of defect taphole bricks but, in practice, these also create problems.
Earlier efforts made at the plant of the assignees of the present inventors to manufacture taphole bricks from material with higher resistance to temperature fluctuations have not led to a solution, because these materials, such as graphite, as a rule possess a much higher heat conductivity. Consequently, very intensive cooling of the taphole brick takes place from the furnace shell or from the surrounding furnace lining, which causes problems in the hardening of the taphole mass.